Surge arresters are useful in protecting electronic circuitry from extreme, over-rating transient currents. These over-rating transients may be caused by switching transients or lightning strikes.
Some surge arresters, especially for higher voltage applications, operate by catastrophic failure of the surge arrester. This is not economically or functionally viable for certain applications, such as secondary, or low voltage applications.
One solution to the design of surge arresters is the use of Metal Oxide Varistors ("MOV"). These MOVs along with surge arresters utilizing them are currently manufactured by many manufacturers. However, the Raychem Corporation of California has developed a very high quality MOV that extends its usefulness for LEC applications. The powder contains smaller particles of the additive metal oxides evenly distributed throughout the larger particles of the primary metal oxide. The use of very fine powder base for the MOV essures a more consistant protection voltage, particularly when sliced to various voltages. Tolerances are held to .+-.0.2%. The following patents assigned to Raychem are representative of the art.
European Patent No. 0,229,464 to Koch et al. (Pub. Jul. 22, 1987) shows a frangible housing for an electrical component reinforced against explosive shattering by wrapping curable sheet material therearound at spaced apart regions. The wrapped material is cured with ultraviolet radiation. This material holds any pieces shattered by over-voltages together.
European Patent No. 0,230,103 to Koch et al. (Pub. Jul. 29, 1987) discloses a surge arrester where circular varistor blocks are stacked for greater voltage applications.
U.S. Pat. No. 5,039,452 to Thompson et al. (August 1991) discloses a process for making ZnO Metal Oxide Varistors (MOV) precursor powder.
PCT Pat. No. WO 91/14304 (GB 91/00405) to Mikli et al. (Pub. Sep. 19, 1991) discloses a surge arrester that has eight varistor blocks stacked together with a fiber-optic cable running through the stack to detect component failure.
PCT Pat. No. WO 93/26017 (U.S. Pat. No. 93/05679) to Wiseman et al. (Pub. Dec. 23, 1993) discloses a method of manufacturing a voltage arrester wherein MOV valve elements are stacked along a longitudinal axis, where the MOV valve elements are compressed between conductive end terminals.
Another solution for low voltage applications of MOV surge arresters is the Wagon Wheel.TM. technology as implemented by LEA Dynatech of Tampa, Fla., and used in the Lightning Eliminators and Consultants, Inc. (LEC) TVSS products. This technology is based on U.S. Pat. No. 4,875,137 to Rozanski et al. (Oct. 1, 1989). The LEC TVSS products utilize low or medium sized, individually fused Metal Oxide Varesistors ("MOV") in parallel.
U.S. Pat. No. 5,519,564 (1996) to Carpenter, Jr. discloses a sandwich block package for mounting a plurality of MOV's in a parallel, spaced apart fashion. A surge is thereby shared by the group of MOVs. Each MOV in the sandwich must be matched to the set for clamping voltage in order to prevent any single MOV from taking the whole surge because it clamped too early.
Ohio Brass Company of Mansfield, Ohio, manufactures a heavy-duty distribution class surge arrester trademarked the Dynavar.RTM. PDV-100. Normal operating voltages are in excess of 2400 volts with clamping voltages around 3000 volts. A single MOV is available packaged in a ribbed EPM rubber housing and epoxy fiberglass wrap cylindrical container. There is no suggestion in the power transmission field to sandwich a single MOV in a sandwich block configuration.
One relevant prior art reference is U.S. Pat. No. 5,088,001 (1992) to Yaworski et al. This device as shown in his FIG. 2 is used for high voltage power distribution and surge protection. An exterior rubber body has a conductive outer layer to transmit surface transients to ground for personnel protection. A pair of thin metal end plates are used to make contact with one or more high voltage MOV's stacked in series, each of which are one inch thick and one and three-quarter inches in diameter. A conductive glue is used to cure surface defects. This glue will decompose at about the same peak current rating of the MOV which is for a 40 mm MOV 30,000 amps (model number V131CA40, CA Series). The structural rigidity is provided by a non-conductive internal epoxy and the exterior rubber shell. The use for the MOV's and the housing is to self destruct without creating a personnel hazard upon a catastrophic surge such as a lightning strike. The application for the device is limited to high voltage or 2,000, up to 33,000 volt primary power distribution systems.
The present invention is new, useful and non-obvious in view of Yaworski and all known prior art because the low voltage (up to 660 volt) secondary power applications create totally different problems. The present invention is designed to handle over 800,000 amperes (rather than 30,000 amperes) and to extend the life of the MOV. The present invention is designed to prevent self destruction and extend its reliability. The present invention is structurally held together with nuts and bolts and optionally has an outer hard plastic box enclosure. The present invention uses novel thin low voltage, high surge current, deposition coated MOV's of large diameters, made especially for up to a 660 volt system for secondary commercial voltages. The present invention maximizes the heat transfer, and low path impedance, to survive a lightning caused current surge, and not self destruct. Yaworski stacks MOV's to handle high voltage requirements because no single MOV as yet handles primary high voltages to 33,000 volts.
Yaworski's high voltage small diameter disks have low impedance due to small diameters. Yaworski's conductive epoxy breaks down at 30,000 amps, but the present invention handles up to 800,000 amps using wide diameter MOV's and deposition coatings in conjunction with "soft" metal disks to make contact between the conductive plates and the MOV. The present invention protects and extends the life of both the circuit and the MOV assembly.
There are several problems with MOV based surge arresters. One problem as illustrated in several of the above patents is that MOVs will often explode when handling excessive transient energy. Compounding this problem is the problem that when MOVs are in parallel, such as with the Carpenter, Jr. '564 patent and Wagon Wheel.TM. technology above, it is possible that the MOVs have different clamping voltages. Thus, a larger than expected proportion of the surge current may flow through a single MOV. This destroys that part of the parallel circuit. The destruction of the single MOV may cause a chain reaction of similar individual MOV overloads, ultimately destroying the entire parallel circuit. In the case of MOVs stacked in series, such failure will cause the entire surge arrester to fail, instead of just degrade.
Prior technologies use wire based connections to, and between the MOVs to increase the energy handling capability of a single assembly. These wires introduce inductance that slows the reaction time and can add some variation in response time. In addition, these wires make a single point contact with the MOV face, thus concentrating the surge energy in a very localized area at the wire connection. This limits the transfer of surge energy between that wire and the MOV; again leading to the major failure mode, burn through at that point, and uneven distribution of the surge energy to the MOV surface. The present invention will eliminate all of the foregoing problems. A single, custom made, relatively large MOV is sandwiched between two contact plates for application in low voltage (up to 660 volt) circuits, using a soft metal disk to reduce contact resistence.
A new casing holds either a single MOV sandwich for single phase applications or a triple (3 phase) MOV sandwich. The casings are pre-mounted on mounting brackets to facilitate installation by the end user on a buss bar or within a dedicated enclosure. The exterior plastic of the casings prevents arcing, short circuits caused by dust and grease build-up across the plates of the MOV sandwich.